Why hydropeaking frequency matters: effects of recurring stranding on fish

Research and management of hydropeaked rivers largely overlook the ecological impacts of recurring flow fluctuations, such as fish stranding, on ecosystem health. This article synthesizes scientific and grey literature, field studies, and experiments to assess the effects of frequent hydropeaking on fish. Findings show that hydropeaking frequency significantly affects the ecological integrity of alpine rivers, with an average of three daily down-ramping events. Despite some evidence of behavioral adaptation of fish to recurrent flow fluctuations, this adaptation appears insufficient to counter the cumulative effect of a series of single hydropeaking events. Larval and juvenile fish are particularly vulnerable, with stranding impacts extending to the population and community levels. Effective mitigation should prioritize reducing the cumulative impact of recurring hydropeaks while ensuring single-event ramping rates and flow amplitudes remain within ecological limits. To effectively safeguard sensitive habitats, targeted mitigation efforts informed by an understanding of habitat dynamics are critical. Furthermore, maintaining lateral connectivity within river systems is essential for supporting resilient fish populations, especially where hydropeaking mitigation possibilities are limited. Finally, this study identifies future research directions on hydropeaking frequency and its ecological effects.